The introduction of a relay station (hereafter referred to as relay node (RN)) into the Long Term Evolution Advanced (LTE-Advanced) of the 3rd Generation Partnership Project (3GPP) is being considered (see Non-Patent Literatures 1 to 3). RN is one of technologies which aim at an increase in the communication speed of a mobile station (hereafter referred to as user equipment (UE)) located at the cell edge, an increase in the cell range of a base station (hereafter referred to as evolved node B (eNB)), or the like. Details of an RN architecture which is being considered in the 3GPP are described in Non-Patent Literature 2.
Hereafter, an outline of a RN system based on the RN architecture described in Non-Patent Literature 2 will be described using FIGS. 1 and 2. FIG. 1 is a diagram showing an example network configuration where an RN of the 3GPP is used. Base stations (eNBs) 91 belongs to a core network (hereafter referred to as CN) 4 of a mobile network operator. The core network (CN) 4 includes a mobility management entity for UE (hereafter referred to as UE MME) 5, a serving gateway/packet data network gateway for UE (hereafter referred to as UE S/P-GW) 6, and an MME for RN (hereafter referred to as RN MME) 97. The UE S/P-GW 6 is a generic name of an S-GW for UE and a P-GW for UE.
The S-GW for UE and P-GW for UE have the functions of routing and forwarding user data (user data packets). The S-GW for UE connects the user plane between the core network 4 and a radio access network to which the base stations (eNBs) 91 and the mobile stations (UEs) 3 belong. The P-GW for UE operates as a gateway that connects the core network 4 with an external network (i.e., packet data network (PDN)). When a relay station (RN) 92 is introduced, the UE S/P-GW 6 maps user data (user packets) about the mobile station (UE) 3 belonging to the relay station (RN) 92 to a logical path (tunnel) provided between the UE S/P-GW 6 and an RN S/P-GW 98 described below and sends the user data to the relay station (RN) 92 through the RN S/P-GW 98.
The UE MME 5 is a node that is responsible for the mobility management and session management (bearer management) of the mobile stations (UE) 3. The UE MME 5 is connected with the base stations (eNB) 91 and the S-GW for UE through the control plane. The UE MME 5 exchanges, with the base stations (eNB) 91 and the S-GW for UE, control signals about an attachment of the UE 3, a handover of the UE 3, and establishment, modification and release of a bearer (evolved packet system (EPS) bearer) for transferring user data between the UE S/P-GW 6 and the UE 3.
The RN MME 97 manages an attachment of the relay station (RN) 92 and a bearer of the relay station (RN) 92.
The radio access network (RAN) 9 includes the base station (eNB) 91, the relay station (RN) 92, and the mobile station (UE) 3. The base station (eNB) 91 generates a base station (eNB) cell 10 and relays traffic between the mobile station (UE) 3 and the core network (CN) 4. The relay station (RN) 92 belongs to the base station (eNB) 91 by a backhaul link (BL1 in FIG. 1), and belongs to the core network (CN) 4 through the backhaul link (BL1). The mobile station (UE) 3 belongs to the base station (eNB) 91 or the relay station (RN) 92 by an access link (AL1 in FIG. 1). The relay station (eNB) 92 generates a relay station (RN) cell 20 and relays traffic between the mobile stations (UE) 3 and the core network (CN) 4. The backhaul link and access link will be described later.
FIG. 2 is a diagram showing bearer mapping when the relay station (RN) 92 and the mobile station (UE) 3 belonging to the relay station (RN) 92 are connected to the network of FIG. 1. As shown in FIG. 2, it is assumed that the functions of S-GW for RN and P-GW for RN are part of the logical function of the base station (eNB) 91 (i.e., these functions are placed in the base station (eNB) 91). However, this only means logical layout of functions, and the base station (eNB) 91 and the functions of S-GW for RN and P-GW for RN do not necessarily need to be implemented in the same hardware. Hereafter, the S-GW for RN and P-GW for RN will be collectively referred to as the RN S/P-GW 98.
The RN MME 97 and RN S/P-GW 98 are network elements which are introduced with the introduction of the relay station (RN) 92. The RN S/P-GW 98 establishes a logical path (tunnel) with the UE S/P-GW 6 and transfers, to the relay station (RN) 92 through the base station (eNB) 91, user data about the mobile station (UE) 3 belonging to the relay station (RN) 92.
When the relay station (RN) 92 belongs to the base station (eNB) 91, a signaling radio bearer (SRB) and a data radio bearer (DRB) for the RN 92 are established between the base station (eNB) 91 and the relay station (RN) 92. The signaling radio bearer for the RN 92 is mapped, by the base station (eNB) 91, to a control protocol (i.e., S1 MME) established between the RN MME 97 and the base station (eNB) 91. That is, with respect to the control plane (i.e., S1 signaling connection) of the relay station (RN) 92, the relay station (RN) 92 is handled as a UE. On the other hand, the data radio bearer for the RN 92 is mapped to a data bearer (S1 bearer) established between the S-GW for RN and the base station (eNB) 91, and is finally terminated by the P-GW for RN. However, as described above, it is assumed that the functions of the RN S/P-GW 98 are placed in the base station (eNB) 91. Accordingly, the data bearer for RN is substantially terminated in the base station (eNB) 91.
When the mobile station (UE) 3 belongs to the relay station (RN) 92, a signaling radio bearer (SRB) and data radio bearer (DRB) for the UE 3 are established between the mobile station (UE) 3 and the relay station (RN) 92. The signaling radio bearer for UE 3 is mapped to the data radio bearer for the RN 92 by the relay station (RN) 92, and is terminated by the UE MME 5 through the RN S/P-GW 98. The data radio bearer for the UE 3 is mapped to the data radio bearer for the RN 92 by the relay station (RN) 92, passes through the RN S/P-GW 98 and the S-GW for UE, and is finally terminated by the P-GW for UE. In other words, control packets transferred between the UE MME 5 and the UE 3 and user data packets transferred between the UE S/P-GW 6 and the UE 3 are both transferred using the data bearer for the RN 92 provided between the RN 92 and the RN S/P-GW 98 (i.e., the data radio bearer between the RN 92 and the eNB 91 and the GTP tunnel between the eNB 91 and the RN S/P-GW 98).
Note that, the 3GPP release 10 assumes that while the relay station (RN) 92 relays the traffic of the mobile station (UE) 3 (in other words, while the RN cell 20 is in operation), the relay station (RN) 92 belongs any one base station (eNB) 91 fixedly and does not change the base station (eNB) to which the relay station (RN) 92 belongs. On the other hand, Non-Patent Literature 3 describes a scenario (mobile RN) in which an RN is mounted on public transportation, such as a train or bus, and thus moves. However, Non-Patent Literature 3 only states that an S1 interface needs to be further considered to realize a mobile RN and does not disclose a specific embodiment.
In the present specification, an eNB having a function of permitting an RN to belong thereto is referred to as “donor eNB (DeNB).” Note that in the present specification, only when describing an event specific to a DeNB which is related to connection with an RN, the term “DeNB” is used to distinguish from a typical eNB. Also, in the present specification, a UE belonging to a DeNB is referred to as “eNB-UE,” and a UE belonging to a RN is referred to as “RN-UE.” In the present specification, when describing an event common to an eNB-UE and an RN-UE, these are simply referred to as “UEs.” In discussions about the 3GPP, a need for supporting a multi-hop RN in the future is being discussed. The term multi-hop RN refers to a technology which allows an RN belonging to a DeNB to be cascaded to another RN. In the present specification, when describing a multi-hop, an RN belonging to a DeNB through a radio interface is referred to as “upper RN,” and an RN belonging to the upper RN through a radio interface is referred to as “lower RN” to distinguish therebetween.
In the present specification, a radio interface between a DeNB and an RN and a radio interface between an upper RN and a lower RN are referred to as “backhaul links.” On the other hand, a radio interface between an eNB and an eNB-UE and a radio interface between an RN and an RN-UE are referred to as “access links.”